Continental drift: Difference between revisions

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{{About|the development of the continental drift hypothesis before 1958|the contemporary theory|plate tectonics|the Russell Banks novel|Continental Drift (novel)}}
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[[File:Antonio Snider-Pellegrini Opening of the Atlantic.jpg|thumb|300px|Antonio Snider-Pellegrini's Illustration of the closed and opened Atlantic Ocean (1858).]]
'''Continental drift''' is the movement of the Earth's [[continents]] relative to each other by appearing to drift across the ocean bed.<ref name=usghistory>[ USGS: Historical perspective on plate tectonics, last updated 19 September 2011]</ref> The speculation that continents might have 'drifted' was first put forward by [[Abraham Ortelius]] in 1596. The concept was independently (and more fully) developed by [[Alfred Wegener]] in 1912. The theory of continental drift was superseded by the theory of [[plate tectonics]], which builds upon and better explains why the continents move.
{{Further|Timeline of the development of tectonophysics}}
===Early history===
[[Abraham Ortelius]] {{Harv|Ortelius|1596}},<ref>{{Citation |last=Romm |first=James |title=A New Forerunner for Continental Drift |journal=Nature |date=February 3, 1994 |volume=367 |pages=407–408 |doi=10.1038/367407a0 |postscript=. |issue=6462|bibcode = 1994Natur.367..407R }}</ref> Theodor Christoph Lilienthal (1756),<ref name=schmeling2004 >{{Cite web |first=Harro |last=Schmeling |url= |title=Geodynamik |year=2004 |publisher=University of Frankfurt |language=German }}</ref> [[Alexander von Humboldt]] (1801 and 1845),<ref name=schmeling2004 /> [[Antonio Snider-Pellegrini]] {{Harv|Snider-Pellegrini|1858}}, and others had noted earlier that the shapes of [[continent]]s on opposite sides of the [[Atlantic Ocean]] (most notably, Africa and South America) seem to fit together.<ref>{{Citation |url= |title=Continents Adrift and Sea-Floors Spreading: The Revolution of Plate Tectonics |first=Stephen |last=Brusatte}}</ref> W. J. Kious described Ortelius' thoughts in this way:<ref>{{Citation |last=Kious |first=W.J. |coauthors=Tilling, R.I. |title=This Dynamic Earth: the Story of Plate Tectonics |origyear=1996 |url= |accessdate=2008-01-29 |edition=Online |publisher=U.S. Geological Survey |isbn=0160482208 |chapter=Historical perspective |chapterurl= |year=2001 |month=February}}</ref>
{{quote|Abraham Ortelius in his work Thesaurus Geographicus ... suggested that the Americas were "torn away from Europe and Africa ... by earthquakes and floods" and went on to say: "The vestiges of the rupture reveal themselves, if someone brings forward a map of the world and considers carefully the coasts of the three [continents]."}}
Writing in 1889, [[Alfred Russel Wallace]] remarks "It was formerly a very general belief, even amongst geologists, that the great features of the earth's surface, no less than the smaller ones, were subject to continual mutations, and that during the course of known geological time the continents and great oceans had again and again changed places with each other."<ref>
{{citation|first=Alfred Russel|last=Wallace|title=Darwinism|year=1889|chapter=12|publisher=Macmillan}}
</ref> He quotes [[Charles Lyell]] as saying "Continents, therefore, although permanent for whole geological epochs, shift their positions entirely in the course of ages"<ref>
{{citation|first=Charles|last=Lyell|title=Principles of Geology|year=1872|edition=11|publisher=John Murray}}
</ref> and claims that the first to throw doubt on this was [[James D. Dana]] in 1849.
In his ''Manual of Geology'', 1863, Dana says "The continents and oceans had their general outline or form defined in earliest time. This has been proved with respect to North America from the position and distribution of the first beds of the Silurian - those of the Potsdam epoch - and this will probably prove to the case in Primordial time with the other continents also".<ref>
{{citation|first=James D.|last=Dana|title=Manual of Geology|year=1863|publisher=Theodore Bliss & Co, Philadelphia}}
</ref> Dana was enormously influential in America - his ''Manual of Mineralogy'' is still in print in revised form - and the theory became known as ''Permanence theory''.<ref>
{{citation|first=Naomi|last=Oreskes|title=Continental Drift|year=2002|url=|archiveurl=|archivedate=Feb 4 2012}}
This appeared to be confirmed by the exploration of the deep sea beds conducted by the [[Challenger expedition]], 1872-6, which showed that contrary to expectation, land debris brought down by rivers to the ocean is deposited comparatively close to the shore in what is now known as the [[continental shelf]]. This suggested that the oceans were a permanent feature of the earth's surface, and did not change places with the continents.<ref>{{harvnb|Wallace|1889}}</ref>
=== Wegener and his predecessors ===
[[File:Alfred Wegener 1910.jpg|thumb|140px|Alfred Wegener]]
The hypothesis that the continents had once formed [[Pangea|a single landmass]] before breaking apart and drifting to their present locations was first presented by [[Alfred Wegener]] to the German Geological Society on 6 January 1912.<ref name=weg>{{Citation
|last= Wegener|first= Alfred |coauthors= |date= 6 January 1912 |title= Die Herausbildung der Grossformen der Erdrinde (Kontinente und Ozeane), auf geophysikalischer Grundlage |journal= Petermanns Geographische Mitteilungen |volume= 63 |issue= |pages= 185–195, 253–256, 305–309 |url= |accessdate= |doi =
|postscript= . }}</ref>
Although Wegener's theory was formed independently and was more complete than those of his predecessors, Wegener later credited a number of past authors with similar ideas:<ref name=wegb>{{Citation | author=Wegener, A. | title = The Origin of Continents and Oceans | year=1929/1966 |publisher=Courier Dover Publications|isbn=0-486-61708-4}}</ref><ref>{{Citation | author=Wegener, A. | title = [[:s:de:Die Entstehung der Kontinente und Ozeane|Die Entstehung der Kontinente und Ozeane]]|edition= 4 | year=1929 |publisher=Friedrich Vieweg & Sohn Akt. Ges.| place=Braunschweig}}</ref>
Franklin Coxworthy (between 1848 and 1890),<ref>{{Citation | author=Coxworthy, F.| title = Electrical Condition or How and Where our Earth was created | place=London | publisher=W. J. S. Phillips| year=1848/1924 | url=}}</ref>
[[Roberto Mantovani]] (between 1889 and 1909), [[William Henry Pickering]] (1907)<ref>{{Citation | author=Pickering, W.H| title = The Place of Origin of the Moon - The Volcani Problems | journal= Popular Astronomy | pages=274–287| year=1907 | bibcode=1907PA.....15..274P}}</ref>
and [[Frank Bursley Taylor]] (1908). [[Eduard Suess]] had proposed a supercontinent [[Gondwana]] in 1858 and the [[Tethys Ocean]] in 1893, from a sunken [[Land_bridge#Land_bridge_theory|land-bridge]]/ [[geosyncline theory]] point-of-view, though. [[John Perry (engineer)|John Perry]] had written an 1895 paper proposing that the earth's interior was fluid, and disagreeing with [[Lord Kelvin]] on the age of the earth.
For example: the similarity of southern continent geological formations had led [[Roberto Mantovani]] to conjecture in 1889 and 1909 that all the continents had once been joined into a [[supercontinent]] (now known as [[Pangaea]]); Wegener noted the similarity of Mantovani's and his own maps of the former positions of the southern continents. Through [[Volcanism|volcanic]] activity due to [[thermal expansion]] this continent broke and the new continents drifted away from each other because of further expansion of the rip-zones, where the oceans now lie. This led Mantovani to propose an [[Expanding Earth theory]] which has since been shown to be incorrect.<ref>{{Citation | author=Mantovani, R.| author-link =| title = Les fractures de l’écorce terrestre et la théorie de Laplace | journal= Bull. Soc. Sc. Et Arts Réunion | pages=41–53| year=1889}}</ref><ref>{{Citation | author=Mantovani, R.| title = L’Antarctide |journal= Je m’instruis. La science pour tous |volume=38 | pages=595–597| year=1909}}</ref><ref>{{Citation | author=Scalera, G. | contribution =Roberto Mantovani an Italian defender of the continental drift and planetary expansion | contribution-url= | editor=Scalera, G. and Jacob, K.-H. |title =Why expanding Earth? – A book in honour of O.C. Hilgenberg | year=2003 | place=Rome | publisher= Istituto Nazionale di Geofisica e Vulcanologia |pages= 71–74}}</ref>
Some sort of continental drift without expansion was proposed by [[Frank Bursley Taylor]], who suggested in 1908 (published in 1910) that the continents were dragged towards the equator by increased lunar gravity during the [[Cretaceous]], thus forming the Himalayas and Alps on the southern faces. Wegener said that of all those theories, Taylor's, although not fully developed, had the most similarities to his own.<ref>{{Citation | author=Taylor, F.B. | title = Bearing of the tertiary mountain belt on the origin of the earth's plan | journal= GSA Bulletin | volume=21 |issue=2| pages=179–226| year=1910 | doi=10.1130/1052-5173(2005)015[29b:WTCCA]2.0.CO;2 | url=}}</ref>
Wegener was the first to use the phrase "continental drift" (1912, 1915)<ref name=weg /><ref name=wegb /> (in German "die Verschiebung der Kontinente" – translated into English in 1922) and formally publish the hypothesis that the continents had somehow "drifted" apart. Although he presented much evidence for continental drift, he was unable to provide a convincing explanation for the physical processes which might have caused this drift. His suggestion that the continents had been pulled apart by the [[Centrifugal force (fictitious)|centrifugal pseudoforce]] (''Polflucht'') of the Earth's rotation or by a small component of astronomical [[precession]] was rejected as calculations showed that the force was not sufficient.<ref>{{cite web |url= |title=Plate Tectonics: The Rocky History of an Idea |quote=Wegener's inability to provide an adequate explanation of the forces responsible for continental drift and the prevailing belief that the earth was solid and immovable resulted in the scientific dismissal of his theories.}}</ref> The [[Polflucht]] hypothesis was also studied by [[Paul Sophus Epstein]] in 1920 and found to be implausible.
==Evidence that continents 'drift'==
[[File:Snider-Pellegrini Wegener fossil map.svg|thumb|right|Fossil patterns across continents ([[Gondwanaland]]).]]
[[Image:Mesosaurus.png|right|thumb|''Mesosaurus'' skeleton, MacGregor, 1908.]]
{{Details|Plate tectonics}}<!--
''Note: This section contains evidence available to Wegener's contemporaries and predecessors'' -->
Evidence for the movement of continents on tectonic plates is now extensive. Similar plant and animal [[fossil]]s are found around different continent shores, suggesting that they were once joined. The fossils of [[Mesosaurus]], a freshwater reptile rather like a small crocodile, found both in [[Brazil]] and [[South Africa]], are one example; another is the discovery of fossils of the land [[reptile]] ''[[Lystrosaurus]]'' from [[Rock (geology)|rock]]s of the same age from locations in [[South America]], [[Africa]], and [[Antarctica]].<ref>{{Cite web |url= |publisher=USGS |title=Rejoined continents [This Dynamic Earth, USGS] }}</ref> There is also living evidence—the same animals being found on two continents. Some [[earthworm]] families (e.g.: Ocnerodrilidae, Acanthodrilidae, Octochaetidae) are found in South America and Africa, for instance.
The complementary arrangement of the facing sides of South America and Africa is obvious, but is a temporary coincidence. In millions of years, [[slab pull]] and [[ridge-push]], and other forces of [[tectonophysics]] will further separate and rotate those two continents. It was this temporary feature which inspired Wegener to study what he defined as continental drift, although he did not live to see his hypothesis become generally accepted.
Widespread distribution of [[Permo-Carboniferous]] glacial sediments in South America, Africa, Madagascar, Arabia, India, Antarctica and Australia was one of the major pieces of evidence for the theory of continental drift. The continuity of glaciers, inferred from oriented [[glacial striation]]s and deposits called [[tillite]]s, suggested the existence of the supercontinent of [[Gondwana]], which became a central element of the concept of continental drift. Striations indicated glacial flow away from the equator and toward the poles, in modern coordinates, and supported the idea that the southern continents had previously been in dramatically different locations, as well as contiguous with each other.<ref name=wegb />
==Rejection of Wegener's theory, and subsequent vindication==
The theory of continental drift was not accepted for many years. One problem was that a plausible driving force was missing.<ref name=usghistory/> And it did not help that Wegener was not a geologist. Other geologists also believed that the evidence that Wegener had provided was not sufficient. It is now accepted that the plates carrying the continents do move across the Earth's surface; ironically one of the chief outstanding questions is the one Wegener failed to resolve: what is the nature of the forces propelling the plates?<ref name=usghistory/>
The British geologist [[Arthur Holmes]] championed the theory of continental drift at a time when it was deeply unfashionable. He proposed that the Earth's mantle contained convection cells that dissipated radioactive heat and moved the crust at the surface. His ''Principles of Physical Geology'', ending with a chapter on continental drift, was published in 1944.<ref>{{Cite book |last=Holmes |first=Arthur |title=Principles of Physical Geology |edition=1st |place=Edinburgh |publisher=Thomas Nelson & Sons |year=1944 |isbn=0-17-448020-2 }}</ref>
[[David Attenborough]], who attended university in the second half of the 1940s, recounted an incident illustrating its lack of acceptance then: "I once asked one of my lecturers why he was not talking to us about continental drift and I was told, sneeringly, that if I could I prove there was a force that could move continents, then he might think about it. The idea was [[wikt:moonshine|moonshine]], I was informed."<ref>{{cite news |title=David Attenborough: force of nature |first=Robin |last=McKie |url= |newspaper=[[The Observer]] |location=London |date=28 October 2012 |accessdate=29 October 2012}}</ref>
Geological maps of the time showed huge land bridges spanning the Atlantic and Indian oceans to account for the similarities of fauna and flora and the divisions of the Asian continent in the Permian era but failing to account for glaciation in India, Australia and South Africa.<ref>See [ map] based on the work of the American paleontologist [[Charles Schuchert]] in {{citation|title=The Science of life|first1=H. G. |last1=Wells|first2=Julian|last2=Huxley|first3=G. P.|last3=Wells|year=1931|page=734|url=}}</ref>
As late as 1953 &ndash; just five years before [[Samuel Warren Carey|Carey]]<ref>{{Cite journal |last1=Carey |first1=S. W. |year=1958 |contribution=The tectonic approach to continental drift |editor1-last=Carey |editor1-first=S. W. |title=Continental Drift&mdash;A symposium |publisher=Univ. of Tasmania |place=Hobart |pages=177–363}}</ref> introduced the theory of [[plate tectonics]] &ndash; the theory of continental drift was rejected by the physicist Scheiddiger on the following grounds.<ref>{{citation|last= Scheidegger |first=Adrian E. |title=Examination of the physics of theories of orogenesis |journal=GSA Bulletin |year=1953|volume= 64|pages= 127–150 |doi=10.1130/0016-7606(1953)64[127:EOTPOT]2.0.CO;2|bibcode = 1953GSAB...64..127S }}</ref>
* First, it had been shown that floating masses on a rotating geoid would collect at the equator, and stay there. This would explain one, but only one, mountain building episode between any pair of continents; it failed to account for earlier orogenic episodes.
* Second, masses floating freely in a fluid substratum, like icebergs in the ocean, should be in [[isostatic]] equilibrium (where the forces of gravity and buoyancy are in balance). Gravitational measurements were showing that many areas are not in isostatic equilibrium.
* Third, there was the problem of why some parts of the Earth's surface (crust) should have solidified while other parts were still fluid. Various attempts to explain this foundered on other difficulties.
Geophysicist [[Jack Oliver (scientist)|Jack Oliver]] is credited with providing seismologic evidence supporting plate tectonics which encompassed and superseded continental drift with the article "Seismology and the New Global Tectonics", published in 1968, using data collected from seismologic stations, including those he set up in the South Pacific.<ref>{{cite news |url= |title=Jack Oliver, Who Proved Continental Drift, Dies at 87 |work=[[The New York Times]] |page=A16 |date=12 January 2011 |accessdate=6 June 2013}}</ref><ref>{{cite journal |last1=Isacks |first1=Bryan |last2=Oliver |first2=Jack |last3=Sykes |first3=Lynn R. |url= |title=Seismology and the New Global Tectonics |date=15 September 1968 |journal=[[Journal of Geophysical Research]] |volume=73 |issue=18 |pages=5855–5899 |doi=10.1029/JB073i018p05855 |accessdate=6 June 2013|bibcode = 1968JGR....73.5855I }}</ref>
It is now known that there are two kinds of crust, [[continental crust]] and [[oceanic crust]]. Continental crust is inherently lighter and of a different composition to oceanic crust, but both kinds reside above a much deeper fluid mantle. Oceanic crust is created at [[Seafloor spreading|spreading centers]], and this, along with [[subduction]], drives the system of plates in a chaotic manner, resulting in continuous [[orogeny]] and areas of isostatic imbalance. The theory of [[plate tectonics]] explains all this, including the movement of the continents, better than Wegener's theory.
* {{Citation |last=Le Grand |first=Homer Eugene |authorlink= |title=Drifting Continents and Shifting Theories |edition= |year=1988 |publisher=Cambridge University |isbn=0-521-31105-5 }}
* {{Citation|first=Naomi|last=Oreskes|title=The Rejection of Continental Drift|publisher=Oxford University Press|year=1999|isbn= 0-19-511732-8|url=}}, (pb: {{ISBNT| 0-19-511733-6}}).
* {{Citation |last=Ortelius |first=Abraham |title=Thesaurus Geographicus |place=Antwerp |publisher=Plantin |year=1596 |language=Latin |edition=3 |ref=CITEREFOrtelius1596 |oclc=214324616 }} (First edition published 1570, [ 1587 edition online])
* {{Citation |title=La Création et ses mystères dévoilés |publisher=Frank and Dentu |place=Paris |first=Antonio |last=Snider-Pellegrini |year=1858 |ref=CITEREFSnider-Pellegrini1858 |url=}}
==External links==
{{Library resources box}}
* [ A brief introduction to Plate Tectonics, based on the work of Alfred Wegener.]
* [ Maps of continental drift, from the Precambrian to the future]
* [ Observe an animation of the breakup of Pangaea] - Animation of continental drift for the last 150 million years
* [ Four main evidences of the Continental Drift theory]
* [ Wegener and his proofs]
{{DEFAULTSORT:Continental Drift}}
[[Category:Plate tectonics]]
[[Category:Geological history of Earth]]
{{Link GA|eo}}

Revision as of 03:32, 11 September 2013

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